This particular mixer offers a usb (2 channel connection) that can be simply plugged into the computer usb 2.0. These channels should be available in Sonar from the channel Input/output drop boxes. You may need to restart Sonar, the mixer or both for them to be recognized. You may also need to run the Wav Profiler in Sonar. Typically the Wave Profiler is found in the Options>Audio menu but may be different on each edition of Sonar. The above info is for Sonar 7.

If you are wanting to record several channels at once then I would recommend connecting your Mixer's Direct Outputs from each channel to a corresponding input of the soundblaster if your soundblaster supports multiple channels. And then use the usb outbut from the Sonar to monitor the computer playback of your master stereo channel. Or, you may connect the soundblaster outputs to unused channels of the mixer or to the tape return or auxilary inputs to monitor the computer playback on the mixer.

Alternately you can send the signal to your soundblaster via the buss outputs. This will limit the number of simaltanious output channels to the number of busses on the mixer, but you can send any mixer channel to any buss without unplugging the instruments or mics.

Here's the link to the page where you can download the free pdf manual for this mixer. Page 14 explains the output connections and has diagrams.
http://www.behringer.com/EN/Products/2442FX.aspx